Characteristics Of GaN-based Light-emitting Diodes Research Articles

Performance Improvement of GaN-Based Light-Emitting Diodes With a Microhole Array, 45° Sidewalls, and a SiO2 Nanoparticle/Microsphere Passivation Layer

The characteristics of GaN-based light-emitting diodes (LEDs) with a hybrid structure incorporating a microhole array, 45 sidewalls, and an appropriate SiO2 nanoparticle (NP)/microsphere (MSs) passivation layer are studied and reported. The use of a SiO2 NP/MSs passivation layer causes a remarkable reduction in reverse-biased leakage current. The employment of this hybrid structure leads to substantial enhancements in optical properties without any degradation in electrical performance. In addition, a lower content of SiO2 NP in the mixed SiO2 NP/MSs solution leads to enhanced optical behavior due to the improved transmittance. Experimentally, as compared with a conventional LED (Device A), the studied Device E shows 50.6%, 50.9%, 48.4%, and 49.9% enhancements in light output power, luminous flux, luminous efficacy, and wall-plug efficiency, respectively. These advantages are mainly attributed to the increased scattering probability and the opportunity to find photon escape cones as well as the reduced total internal reflection and Fresnel reflection effects. Therefore, the studied hybrid structure provides a promise for high-performance GaN-based LED applications.

Forward and reverse current aging of GaN-based light-emitting diodes fabricated with Ag-based reflective electrodes

In this study, the effects of forward and reverse current aging on the reliability characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag-based reflective electrodes were investigated. The LEDs showed a noticeable degradation in output after forward current (100 mA) or reverse current (−0.1 mA) aging for 24 h. This could be due to the significantly increased leakage currents caused by disruptions of Ni/Ag/Pt reflective electrodes via Ag migration after forward current aging (causing surface leakage), or via electrode destruction near the V-defect region after reverse current aging (causing bulk leakage). Notably, this degradation behavior could be explained in terms of current crowding.

Performance Enhancement of GaN-Based Light-Emitting Diodes by Using Transparent Ag Metal Line Patterns

Characteristics of GaN-based light-emitting diodes (LEDs) with 1-D stripe (B-series) and 2-D grid (A-series) Ag metal line patterns are comprehensively studied and reported. Due to the enhanced current spreading capability, as compared to a conventional LED, the significantly reduced forward voltages (>500 mV) of studied A- and B-series devices are obtained under the forward current of 20 mA. Particularly, a very low current coefficient of junction temperature $\text{T}_{\text {j}}$ (~0.03°/mA) could be obtained for studied A- and B-series devices which is remarkably superior to the conventional device (0.61°/mA). It means that the undesired thermal effect is nearly negligible in studied devices. Experimentally, based on the less absorption effect of photons from Ag metal lines, the 1-D stripe design (B-series) shows better optical properties than 2-D grid one (A-series). Therefore, based on the appropriate design of transparent 1-D Ag pattern, the improved performance of GaN-based LEDs, including smaller forward voltage, higher light output power, higher external quantum efficiency, higher wall-plug efficiency, and negligible current dependence (wider current operating regime) could be simultaneously attained.

Improved Reliability of InGaN-Based Light-Emitting Diodes by HfO2 Passivation Layer.

We utilized a passivation layer to improve the leakage current and reliability characteristics of GaN-based light-emitting diodes. The electrical and optical characteristics of the fabricated LEDs were characterized by current-voltage and optical power measurements. The HfO2 passivation layer showed no optical power degradation and suppressed leakage current. The low deposition temper- ature of sputtered HfO2 is responsible for the improved reliability of the LEDs because it suppresses the diffusion of hydrogen plasma into GaN to form harmful Mg-H complexes.

Electrical Characteristics of GaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.

Fabrication and characteristics of GaN-based light-emitting diodes with a reduced graphene oxide current-spreading layer.

A reduced graphene oxide (GO) layer was produced on undoped and n-type GaN, and its effect on the current- and heat-spreading properties of GaN-based light-emitting diodes (LEDs) was studied. The reduced GO inserted between metal electrode and GaN semiconductor acted as a conducting layer and enhanced lateral current flow in the device. Especially, introduction of the reduced GO layer on the n-type GaN improved the electrical performance of the device, relative to that of conventional LEDs, due to a decrease in the series resistance of the device. The enhanced current-spreading was further of benefit, giving the device a higher light output power and a lower junction temperature at high injection currents. These results therefore indicate that reduced GO can be a suitable current and heat-spreading layer for GaN-based LEDs.

Ideality factor of GaN-based light-emitting diodes determined by the measurement of photovoltaic characteristics

We present a method for determining the ideality factor of GaN-based light-emitting diodes (LEDs) by using the measured photovoltaic characteristics. The relation between the short-circuit current and the open-circuit voltage is obtained as the incident power of a laser diode emitting at 405 nm is varied, which is used to determine the ideality factor of the LED. From the photovoltaic measurements, the ideality factors of a blue and a green LED are determined to be 1.16 and 1.78, respectively. The ideality factors obtained by using the photovoltaic measurement are found to be much smaller than those obtained by using the I–V curve without illumination, which is believed to result from the different carrier generation and transport mechanisms. Investigating the photovoltaic characteristics of GaN-based LEDs is expected to provide insight into the origin of the high diode ideality factor in GaN-based devices.

Temperature-dependent photoluminescence in light-emitting diodes.

Temperature-dependent photoluminescence (TDPL), one of the most effective and powerful optical characterisation methods, is widely used to investigate carrier transport and localized states in semiconductor materials. Resonant excitation and non-resonant excitation are the two primary methods of researching this issue. In this study, the application ranges of the different excitation modes are confirmed by analysing the TDPL characteristics of GaN-based light-emitting diodes. For resonant excitation, the carriers are generated only in the quantum wells, and the TDPL features effectively reflect the intrinsic photoluminescence characteristics within the wells and offer certain advantages in characterising localized states and the quality of the wells. For non-resonant excitation, both the wells and barriers are excited, and the carriers that drift from the barriers can contribute to the luminescence under the driving force of the built-in field, which causes the existing equations to become inapplicable. Thus, non-resonant excitation is more suitable than resonant excitation for studying carrier transport dynamics and evaluating the internal quantum efficiency. The experimental technique described herein provides fundamental new insights into the selection of the most appropriate excitation mode for the experimental analysis of carrier transport and localized states in p-n junction devices.

Enhanced performance of GaN-based light-emitting diodes by using a p-InAlGaN/GaN superlattice as electron blocking layer

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) with various kinds of electron blocking layers (EBLs) are analyzed numerically. The results indicate that an enhanced hole injection efficiency and a reduced electron leakage could be achieved with the GaN-based LED where a p-InAlGaN/GaN superlattice (SL) was employed as EBL as compared with the conventional GaN-based LEDs using rectangular p-AlGaN EBL or p-AlGaN/GaN SL EBL. Moreover, it was found that the efficiency droop could be significantly improved at high injection current density for GaN-based LEDs with p-InAlGaN/GaN SL EBL.

Effects of Initial GaN Growth Mode on Patterned Sapphire on the Opto-Electrical Characteristics of GaN-Based Light-Emitting Diodes

We have studied the initial growth modes of GaN on patterned sapphire substrate (PSS) with different initial TMGa flow rates. The FWHM of the (102) XRD spectrum of GaN on PSS increased from 470 to 580 arcsec when the initial TMGa flow rate was increased from 80 to 200 sccm. A low TMGa flow rate sufficiently suppresses GaN island growth on the top of the pattern and hence improves GaN crystal quality. The electrical and optical characteristics of GaN-based LEDs on PSS with low initial TMGa were also improved. More than 90% of the GaN LED chips with low initial GaN growth rate can hold the 1-kV machine-mode electrostatic discharge level.

Effects of ITO Pattern on the Electrical and Optical Characteristics of LEDs

We studied on the effects of patterned indium-tin oxide (ITO) on the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) in detail. In our experiment, LEDs with varied patterned ITO layers are fabricated. The electrical and optical characteristics are analyzed through the diode equation and 3D electro-thermal simulation, the rate equation and Monte-Carlo ray-tracing simulation, respectively. The analysis results indicate that the light output power (LOP) of the LEDs with patterned ITO achieve an enhancement of 12.7% compared to that of the LEDs with planar ITO. Moreover, the series resistance and ideality factor are the minimum when ITO is planar, and will increase with spacing of ITO pattern decreasing due to the degeneration of current spreading and ohmic contact. However, the light extraction efficiency (LEE) is smaller for LEDs with planar ITO and will increase with spacing decreasing, which is due to the increase of probability of photon redirecting and escaping. These analysis results are helpful to understand the effects of ITO pattern on electrical and optical characteristics and therefore to design an optimal pattern structure.

GaN-Based Light-Emitting Diode With Sputtered AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN-based light-emitting diodes (LEDs) were improved using a sputtered AlN nucleation layer. Replacing the in situ AlN nucleation layer with the sputtered AlN nucleation layer reduced the (002) and (102) X-ray rocking curve widths of the GaN layer from 318.0 to 201.1 and 412.5 to 225.0 arcsec, respectively. The -20-V reverse leakage current of the LEDs with the sputtered AlN nucleation layer is about three orders less than that of the LEDs with the in situ AlN nucleation layer. In addition, the LEDs with sputtered AlN nucleation layer could sustain more than 60% passing yield on the ESD test of under a -600-V machine mode, whereas the LEDs with the in situ AlN nucleation layer sustained less than 40% passing yield. Moreover, the 20-mA output power of the LEDs with the sputtered AlN nucleation layer also improved by approximately 5.73% compared with that of the LEDs with the in situ AlN nucleation layer.

Electrical and optical characteristics of GaN-based light-emitting diodes fabricated with emission wavelengths of 429–467 nm

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with emission wavelengths of 429–467nm were investigated. The optical output increased with increasing emission wavelength, which is attributed to the enhanced quantum confinement effect as a result of indium composition fluctuation. With higher indium content, the LEDs exhibited unfavorable performance, including a larger efficiency droop, spectral blueshift, and spectral broadening, due to indium-induced strains. The effect of heterointerfaces associated with the indium content of the active region on the device resistance was negligible.

Performance characteristics of GaN-based light-emitting diodes fabricated with AgNi, AgCu, and AgAl-alloy reflectors

We report on the performance characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag-alloy p-type reflectors including AgNi, AgCu, and AgAl. Compared to the reference LEDs fabricated with Ag, LEDs fabricated with AgNi and AgCu produced smaller forward voltages and higher light output power. Under optimized thermal annealing conditions, the forward voltages obtained were 3.56, 3.29, 3.28, and 3.44 V, the light output powers were 14.38, 15.73, 14.40, and 12.77 mW, and the power efficiencies were 20.2%, 23.91%, 21.95%, and 18.45% for LEDs fabricated with Ag, AgNi, AgCu, and AgAl, respectively. The surface morphology of Ag-alloy was also found to be smoother than that of Ag, suggesting that Ag-alloy reflectors, particularly AgNi, are very promising for practical applications.

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.

Investigation of the Electrostatic Discharge Performance of GaN-Based Light-Emitting Diodes With Naturally Textured p-GaN Contact Layers Grown on Miscut Sapphire Substrates

The electrostatic discharge (ESD) characteristics of GaN-based light-emitting diodes (LEDs) with naturally textured p-GaN contact layers grown on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis miscut sapphire substrates are studied and demonstrated. Based on the machine model, the device grown on a 0.35° miscut sapphire shows the highest ESD tolerance, whereas the device grown on a 0.2° miscut sapphire exhibits the poorest tolerance. It is found that this phenomenon is primarily related to the presence of maximum capacitance Cm values rather than the difference in defect densities between LEDs. The variation in Cm values is caused by the parasitic capacitance effect induced by different p-GaN surface morphologies between the studied devices. This observation gives us a more reliable application in improving the ESD performance based on the device grown on a 0.35° miscut sapphire.

Complete suppression of surface leakage currents in microperforated blue light-emitting diodes

We investigated the effects of thermal annealing and sulfide passivation on the electrical characteristics of GaN-based light-emitting diodes (LEDs) whose active regions were integrated with a plasma-etched microhole array resembling a photonic crystal structure. Thermal annealing removed most plasma damage in the near-surface bulk region, whereas (NH4)2S treatment only passivated the defect states at the immediate surface, each producing a partial recovery of the electrical characteristics. It was found that annealing at 700 °C used in conjunction with prolonged sulfide passivation eliminated all the effects of plasma damage and resulted in a complete suppression of surface leakage in the microperforated LEDs.

Improvement in the characteristics of GaN-based light-emitting diodes by inserting AlGaN-GaN short-period superlattices in GaN underlayers

We report the influence of short-period superlattice (SPSL)-inserted structures in the underlying undoped GaN on the characteristics of GaN-based light-emitting diodes (LEDs). The measurements of current-voltage (I-V) curves indicate that GaN-based LEDs having pseudomorphic Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures exhibit improvements in device characteristics with the best LED being inserted with two sets of five-pair Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL structure. Based upon the results of etch pit counts, double-crystal X-ray diffraction measurements and transmission electron microscopic observations of the GaN-based LEDs, it was found that the Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures tended to serve as threading dislocation filters in the LEDs so that the improved I-V characteristics were achieved.

Improvement of the electrical performance of near UV GaN-based light-emitting diodes using Ni nanodots

We report on the improvement of the electrical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag (1 nm)/indium tin oxide (ITO) (250 nm) p-type contacts by using Ni nanodots. As-deposited Ag/ITO contacts with and without the Ni nanodots give non-linear electrical behaviour. However, annealing the samples at 530–630 °C for 1 min in air results in ohmic behaviour. The reverse leakage current characteristics of near UV LEDs are much improved when the Ni nanodots are used. The output power (at 20 mA) of LEDs fabricated with the Ni nanodots is enhanced by a factor of 1.34 as compared with that of LEDs made without the Ni nanodots.

Improved characteristics of GaN-based light-emitting diodes by distributed Bragg reflector grown on Si

We report the improved characteristics of GaInN light-emitting diodes (LEDs) on Si substrates by a distributed Bragg reflector (DBR). The DBR-based GaInN multi quantum-well LED structures were grown on n-Si (111) substrates using a conventional horizontal metalorganic chemical vapor deposition method. The number of Al0.3Ga0.7N/AlN pairs in the DBR was changed from 1 to 5. The measured PL peak intensity ratio agrees well with the calculated value. In an EL measurement, the output power increases with an increase in the number of pairs in the DBR (less than 3 pairs). The light output power of a 3-pair DBR-based LED is approximately twofold larger than that of a non-DBR-based LED. However, it significantly decreases in a 5 pair of DBR, because of a crack formation. Although the suppression of crack formation remains a problem, the DBR is still very promising for the fabrication of high-performance LEDs on Si. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)